EP3833593A1

EP3833593A1 - Steering-rack-force optimised steering sensation of a steer-by-wire motor vehicle steering system

Info

Publication number: EP3833593A1
Application number: EP19769010.0A
Authority: EP
Inventors: Manuel Rohrmoser
Original assignee: ThyssenKrupp AG; ThyssenKrupp Presta AG
Current assignee: ThyssenKrupp AG; ThyssenKrupp Presta AG
Priority date: 2018-08-08
Filing date: 2019-08-06
Publication date: 2021-06-16
Anticipated expiration: 2039-08-06
Also published as: DE102018119268A1; US20210221431A1; EP3833593B1; CN112601691B; WO2020030621A1; CN112601691A; DE102018119268B4; US11807291B2

Abstract

The invention relates to a method for adapting a steering torque for controlling a feedback actuator (4) of a steer-by-wire motor vehicle steering system having the following steps: * providing a basic steering torque (T_b), * providing a steering torque (T_r) which is based on the steering rack force, * calculating a difference between the basic steering torque (T_b) and the steering torque (T_r) which is based on the steering rack force, * and if the difference exceeds a predefined limiting value, adapting the basic steering torque (T_b) to the steering torque (T_r) which is based on the steering rack force and passing on the resulting steering torque for controlling the feedback actuator (4), * and if the difference remains below the predefined limiting value, passing on the basic steering torque (T_b) for controlling the feedback actuator.

Description

Rack force optimized steering feel of a steer-by-wire vehicle steering

The present invention relates to a method for adapting a

Steering torque for controlling a feedback actuator of a steer-by-wire motor vehicle steering system with the features of the preamble of claim 1 and a method for controlling a steer-by-wire steering system with the features of the preamble of claim 9, and a steer-by -Wire steering system with the features of the preamble of claim 10.

In steer-by-wire steering systems, the position of the steered wheels is not directly coupled to the steering input means, for example a steering wheel.

There is a connection between the steering wheel and the steered wheels via electrical signals. The driver's steering request is picked up by a steering angle sensor and, depending on the driver's steering request, the position of the steered wheels is regulated via a steering actuator. A mechanical connection to the wheels is not provided, so that no immediate force feedback is transmitted to the driver after the steering wheel is actuated. However, a correspondingly adapted feedback, for example when parking or when driving straight ahead, in which one is adapted to the vehicle reaction, differs depending on the vehicle manufacturer

Steering torque is desired as force feedback is provided. When cornering, reaction forces act as lateral forces on the steering gear, which the feedback actuator simulates in the form of a torque opposite to the steering direction. The driver experiences a steering feeling that can be predetermined thereby. In order to simulate the effects of the road on the steering wheel in steer-by-wire steering, it is necessary to use the steering wheel or the

Steering column to provide a feedback actuator (FBA), which in

Depends on the desired repercussions of the steering handle.

Conventionally, in electric power steering systems (EPS), the feedback properties of the steering are determined by the rack force that acts on the rack from the tie rods, which are connected to the wheels via the chassis. The rack force is significantly influenced by the current cornering forces. A substantial part of the current rack force corresponds to one

Lateral acceleration. The rack force is not only determined by the lateral forces that occur while driving through a curve, but a number of other variables of a current driving situation have an influence on the rack force. An example of this is the

Road conditions (bumps, ruts, coefficient of friction).

The currently applied rack force can be determined by means of a torque sensor arranged on the rack or by estimation using a so-called observer based on a model of the steering system. Such a method is disclosed, for example, in DE 103 320 23 A1. To determine a steering torque for the EPS steering system of a vehicle, the steering torque is determined as a function of the lateral force occurring on steered wheels or as a function of the actual steering torque. The known method provides that

To estimate or model lateral force by means of a sensor or on the basis of a model of the steering of the vehicle as a function of at least one of the variables lateral acceleration, steering angle and vehicle speed. This model proves to be disadvantageous because it does not take into account other disturbing influences, such as road conditions, and therefore does not have the desired accuracy.

In steer-by-wire steering systems, an artificially generated steering torque is currently used to control the feedback actuator. It is very complex to map the road feedback and the steering feeling in and above a dynamic driving range.

It is therefore an object of the present invention to provide a method for

Adaptation of a steering torque for controlling a feedback actuator of a steer-by-wire motor vehicle steering system and a method for controlling a steer-by-wire steering system for motor vehicles, which allow a more accurate mapping of a realistic steering feel. Furthermore, a steer-by-wire steering system is to be specified that enables improved steering behavior.

This object is achieved by a method for adapting a steering torque for controlling a feedback actuator of a steer-by-wire motor vehicle steering system with the features of claim 1, and a method for controlling a steer-by-wire steering system for motor vehicles with the features of claim 9 and a steer-by-wire steering system for motor vehicles with the features of claim 10 solved. Advantageous developments of the invention are mentioned in the subclaims.

Accordingly, a method for adapting a steering torque for controlling a feedback actuator of a steer-by-wire motor vehicle steering is provided with the following steps:

Providing a basic steering torque,

Provision of a steering torque based on rack force,

Forming a difference between the basic steering torque and the steering torque based on rack force,

• If the difference exceeds a specified limit, Adapting the basic steering torque to the steering torque based on the rack force and passing on the resulting steering torque to control the feedback actuator,

• If the difference remains below the specified limit value, the basic steering torque is passed on to control the feedback actuator.

By adapting the basic steering torque when the deviation is too great, the resulting steering feel becomes very realistic without losing the advantages of a basic steering feel.

The limit value is preferably formed by forming an uncertainty band around the steering torque based on rack force. It is advantageous if the uncertainty band is symmetrical to the value of the steering torque based on the rack force. The uncertainty band is stretched up and down at regular intervals from the basic steering torque. This firmly defined uncertainty band forms like a shell around the basic steering torque.

In an advantageous embodiment, the uncertainty band has an upper band limit of 125% of the value based on the toothed rack force

Steering torque and lower band limit of 75% of the value of the

Gear rack force based on steering torque. However, it can also be provided that the upper band limit is 115% of the value of the steering torque based on rack force and the lower band limit is 85% of the value of the steering torque based on rack force.

The band gap between the two band boundaries of the uncertainty band is preferably at least 20% and at most 60% of the value of the steering torque based on the rack force.

The basic steering torque is preferably generated from a vehicle speed and a steering angle applied to the steering wheel and is thereby for

High friction roads designed. This enables the driver to be given an optimized steering feel, which is electromechanical Steering system corresponds. To generate the basic steering torque, further parameters can preferably be incorporated, such as a steering wheel speed, a rack position or a rack speed.

The racking force-based steering torque is preferably determined by means of an estimate of the racking force, the vehicle speed and the measured rack position. The rack force is preferably determined using a steering gear model-based estimate. This preferably has a non-linear steering gear model. The steering gear model-based estimate is based on rack information, such as the rack position and a torque query, and is independent of roadway information. The steering gear model-based estimate can further be expanded to include a vehicle model-based estimate in order to improve the quality of the feedback from the road to the driver and thus the steering feel. In this case, an estimated steering gear model-based toothed rack force is determined in one unit and an estimated linear vehicle model-based one in another unit

Rack force, in which roadway information is implemented, is determined. The values of both rack forces are compared with each other, if necessary adapted to the determined vehicle model-based rack force, and a resulting rack force is determined from this. The determined or adapted resulting rack force is then converted into the rack force-based steering torque. The further adjustment of the steering torque is carried out as described above

Comparison between the basic steering torque and the steering torque based on rack force.

The adaptation preferably comprises a scaling of the difference, which is then given as an offset to the basic steering torque. Scaling is preferably carried out in a state when the driver is oversteering or understeering.

Furthermore, a method for controlling a steer-by-wire steering system for a motor vehicle comprises: an electronically controllable steering actuator acting on the steered wheels,

- a control unit,

a feedback actuator which can be acted upon by a driver with a driver request for a steering angle via a steering input device and which outputs a feedback signal to the steering input device in response to the driver request and a driving state of the motor vehicle,

a signal transmission that transmits the driver's request to the control unit,

the control unit controls the steering actuator in order to transform the driver's wish into a deflection of the steered wheels,

- Provided, the feedback signal depending on a

Steering torque takes place and the steering torque is determined using a previously described method.

A steer-by-wire steering system for a motor vehicle is also comprehensive:

- An electronically controllable acting on the steered wheels

Steering actuator,

- a control unit,

a device for signal transmission, which transmits the driver's request to the control unit, is provided, wherein the control unit controls the steering actuator in order to transform the driver's request into a deflection of the steered wheels, and wherein the steer-by-wire steering system is set up to carry out a previously described method. In particular, the control unit, which has an evaluation unit, executes the method.

Preferred embodiments of the invention are explained in more detail below with reference to the drawings. Components of the same type or having the same function are designated in the figures with the same reference symbols. It

demonstrate :

FIG. 1: a schematic representation of a steer-by-wire steering system,

FIG. 2: a block diagram of a control of the steer-by-wire

Steering system,

Figure 3 is a block diagram of a module for adapting a

Steering torque,

Figure 4: a time course of a basic steering torque, a steering torque calculated from a rack force and a resulting steering torque, and

Figure 5: another block diagram of a module for adapting the

Steering torque, which is expanded by a vehicle model.

A steer-by-wire steering system 1 is shown in FIG. A steering angle sensor (not shown) is attached to a steering shaft 2 and detects the driver steering angle applied by turning a steering input means 3, which in the example is designed as a steering wheel. However, a steering torque can also be recorded. A joystick can serve as steering input. Furthermore, a feedback actuator 4 is attached to the steering shaft 2, which serves to simulate the reactions from the roadway 70 to the steering wheel 3 and thus to give the driver feedback on the steering and driving behavior of the vehicle. The driver's steering request is made via the rotation angle a of the steering shaft 2 measured by the rotation angle sensor via signal lines to a feedback actuator monitor unit 10 transmitted, as illustrated in Figure 2. The feedback actuator monitor unit 10 transmits the driver's steering request to the control unit 60. The feedback actuator monitor unit 10 also preferably controls the feedback actuator 4. The feedback actuator monitor unit 10 can also be formed integrally with the control unit 60. The

Control unit 60 controls an electric steering actuator 6, which controls the position of the steered wheels 7, as a function of the signal from the angle of rotation sensor and further input variables. The steering actuator 6 acts indirectly on the steered wheels 7 via a steering rack steering gear 8, such as a rack and pinion steering gear, as well as via tie rods 9 and other components.

FIG. 2 shows a control of the feedback actuator 4. The feedback actuator 4 receives signals via a signal line 50, among other things, from the rotation angle sensor, which measures and stores the steering angle α, the steering angle acceleration and the steering angle speed. The feedback actuator 4 communicates with a feedback actuator monitor unit 10, which controls the feedback actuator 4. The feedback actuator monitor unit 10 receives the actual wheel steering angle β of the steered wheels 7 from a control unit 60 of the steering actuator 6, as well as other variables that the

Control unit 60 has determined. The rack position 120 measured on a rack 12 and further roadway information 13 are passed on to the control unit 60. The control unit 60 has a module 14 for determining the rack force. Module 14 for

Determination of the rack force uses a steering gear model-based rack force F _{r eStrack} , which is estimated on the basis of the measured rack position 120 and the steering torque initiated by the driver. The determined rack force 14 is used in an evaluation unit 140

Adjustment of a basic steering torque used. The feedback actuator 4 is controlled in accordance with the resulting steering torque, which thus generates a steering feeling. The control unit 60 also receives driver-side steering commands 51, such as the steering angle status.

The evaluation unit 140 is shown in detail in FIG. On

Base steering torque T _b is in a unit 30 from the vehicle speed v _{Veh U} nd an applied on the steering wheel steering angle or a steering wheel generates speed and designed for high friction coefficient road, ie it is assumed that the road friction coefficient of the road surface having a high coefficient of friction so as to emulate an electromechanical steering system. A high coefficient of friction arises, for example, on dry asphalt, while a medium coefficient of friction arises in snow and a low coefficient of friction occurs in ice.

A steering feeling resulting from the basic steering torque T _b has no discontinuities and is therefore very pleasant for the driver. However, it is very complex to map road feedback (coefficient of friction and the like).

In addition to the basic steering torque T _b , a steering torque T _r based on a toothed rack force is provided. This steering torque T _r , which is based on rack force, is determined in a unit 32 by means of an estimate of the steering gear model-based rack force F _{r eStrack} , the vehicle _speed v _{; Ve} h and the measured rack position s _{r meas} and, if appropriate, further parameters.

In order to incorporate the road feedback (ruts, bumps, road friction, etc.) into the steering feel, the basic steering torque T is compared according to the invention by the rack-and-pinion steering torque T _r and if necessary, especially if oversteer or oversteer

Understeer is present, scaled.

In a first step, the basic steering torque T and the steering force T _r based on the rack force are compared by forming the difference 15.

The difference between the two values DT flows into a scaling 16. Since the steering force T _r based on the rack force brings certain uncertainties with it, a dynamically generated uncertainty band is placed around the steering force T _{r based} on the rack force. The respective value of the steering torque T _r formed on the basis of the rack force is reduced by +/- 25%, preferably +/- 15% (lower band limit) or increased (upper

Band limit). In addition to the difference value DT, the

Information about the uncertainty 17 and the rack-and-pinion-based steering torque T _r into the scaling 16. In the scaling 16 is a scaled difference T _{diff SCaied} from the differential torque DT and the rack-force-based steering torque T _r for the base steering torque T _b , which is applied to the base torque as offset 18, the resulting steering torque T _res for controlling the steer-by Wire steering is used.

FIG. 4 shows the course over time of a basic steering torque T _b , an associated steering torque T _r based on rack force and the resulting steering torque T _res . The scaling is done as follows:

If the basic steering torque T is within the upper and lower band limits 19, 20 of the uncertainty band of the steering torque T _r formed on rack force, the basic steering torque T is not adapted and the resulting steering torque T _res corresponds to the basic steering torque T. If, on the other hand, the basic steering torque T lies outside the uncertainty band, this is adapted to the steering torque T _{r based} on the rack force. The adjustment is preferably carried out depending on the driving situation

Scaling the difference between the steering torque T _r based on the rack force and the differential torque DT and is then added as an offset to the basic steering torque T, ie by dead band areas on the uncertainty band. In the event of an understeer or oversteer, the complete difference is added, so that the steering feel corresponds exclusively to the modeled rack-and-pinion-based steering torque T _r . In the case of roads with a low coefficient of friction or driving maneuvers in the limit area, the basic steering torque T is modified accordingly in order to produce a realistic steering feel.

The resulting steering torque T _res flows into the control of the feedback actuator. The resulting steering torque T _res is therefore the arithmetic steering torque which is intended to counteract the steering torque introduced into the steering wheel by the driver.

FIG. 5 shows an alternative embodiment of the evaluation unit 140, which is expanded by a linear vehicle model in which roadway information is implemented. Here, as in FIG. 3, the basic steering torque T in the unit 30 becomes the vehicle speed v _{; Ve} h and one on the steering wheel generated steering angle or a steering wheel speed and designed for high-friction roads.

In addition to the basic steering torque T _b , a steering torque T _r based on a toothed rack force is provided. The racking force-based steering torque T _r is composed of a racking force that results from a linear vehicle model-based estimate of the racking force F _{r eStvehrack} and the steering model-based racking force F _{r eStrack} . The vehicle model-based rack force is determined in a unit 28 and the steering gear model-based rack force is determined in a unit 29 from the vehicle speed v _Veh and the measured rack position s _{r meas} . The determined rack forces are compared in unit 31 and an estimated rack force is formed from both values. Furthermore, the steering force T _r based on the rack force is formed in the unit 31 from the estimated rack force. The resulting steering torque T _res is determined as described in FIG. 3.

Claims

claims

1. Method for adjusting a steering torque to control a

Feedback actuator (4) of a steer-by-wire vehicle steering system with the following steps:

Providing a basic steering torque (T _b ),

Providing a steering torque (T _r ) based on rack force, characterized in that the method comprises further steps:

Forming a difference between the basic steering torque (T _b ) and the steering torque (T _r ) based on rack force,

If the difference exceeds a predetermined limit value, adapting the basic steering torque (T) to the steering torque (T _r ) based on rack force and passing on the resulting steering torque to control the feedback actuator (4),

• If the difference remains below the specified limit value, the basic steering torque (T) is passed on to control the feedback actuator.

2. The method according to claim 1, characterized in that the

Limit value is formed by forming an uncertainty band around the steering torque (T _r ) based on rack force.

3. The method according to claim 2, characterized in that the

Uncertainty band is symmetrical to the steering torque (T _r ) based on rack force.

4. The method according to claim 3, characterized in that the

Uncertainty band an upper band limit of 125% of the value of the Rack and pinion force based steering torque (T _r ) and a lower one

Band limit of 75% of the value of the steering torque based on rack force (T _r ).

5. The method according to claim 4, characterized in that the upper band limit at 115% of the value of the rack force based

Steering torque (T _r ) and the lower band limit is 85% of the value of the steering torque (T _r ) based on rack force.

6. The method according to any one of claims 2 to 4, characterized in that the band gap between the two band boundaries of the

Uncertainty band is at least 20% and at most 60% of the value of the steering torque (T _r ) based on rack force.

7. The method according to any one of the present claims, characterized in that the basic steering torque (T _b ) is generated from a vehicle speed (v _veh ) and a steering angle (a) applied to the steering wheel and is designed for high-friction roads.

8. The method according to any one of the present claims, characterized in that the steering rack force-based steering torque (T _r ) is determined by means of an estimated rack force (F _{r eStrack} ), the vehicle speed (v _veh ) and the measured rack position (s _{r mea} s) ,

9. Method for controlling a steer-by-wire steering system for a

Motor vehicle comprising:

- an electronically controllable steering actuator (6) acting on the steered wheels (7), - a control unit (60),

- A feedback actuator (4) which can be acted upon by a driver with a driver request for a steering angle (a) via a steering input means (3) and a feedback signal to the Outputs steering input means (3) in response to the driver's request and a driving state of the motor vehicle,

a signal transmission which transmits the driver's request to the control unit (60),

- The control unit (60) controls the steering actuator (6) in order to transform the driver's request into a deflection of the steered wheels (7),

- And wherein the feedback signal is dependent on a steering torque, characterized in that the steering torque is determined by means of a method according to one of claims 1 to 8.

10. Steer-by-wire steering system (1) for a motor vehicle, comprising:

- An electronically controllable steering actuator (6) acting on the steered wheels (7),

- a control unit (60),

a feedback actuator (4) which can be acted upon by a driver with a driver's request for a steering angle via a steering input means (3) and outputs a feedback signal to the steering input means in response to the driver's request and a driving state of the motor vehicle,

- A device for signal transmission that transmits the driver's request to the control unit (60), the control unit (60) controls the steering actuator (6) in order to convert the driver's request into a deflection of the steered wheels (7)

transform, characterized in that the steer-by-wire steering system (1) is set up to carry out a method according to one of claims 1 to 9.